The present invention relates to a process for the preparation of block copolymers for a sheet of film comprising a styrene-type hydrocarbon (hereinafter referred to as "styrene") and a conjugated diene hydrocarbon (hereinafter referred to as "diene").
It is well known that various types of block copolymers of styrene and diene units can be obtained with use of an organolithium compound as a polymerization initiator. For instance, U.S. Pat. No. 3,287,333 discloses a process for producing block copolymers from a mixture of a conjugated diene and a vinyl-type aromatic hydrocarbon with Li-initiators. Also, U.S. Pat. No. 3,629,387 teaches rubber-modified, impact-resistant polystyrene films made of a block copolymer of styrene and conjugated diene units.
Among the block copolymers, the present invention is particularly directed to a resinous block copolymer for a sheet or film containing 70 to 85% by weight of styrene units.
Processes for the preparation of block copolymers of styrene and diene with use of organolithium compounds as a polymerization initiator may be classified broadly into the two types:
1. POLYMERIZING IN REGULAR SEQUENCE STYRENE AND DIENE MONOMERS.
2. POLYMERIZING A MIXTURE OF STYRENE AND DIENE MONOMERS IN A NON-POLAR SOLVENT TO OBTAIN COPOLYMERS SIMILAR TO PERFECT BLOCK COPOLYMERS OBTAINED IN METHOD (1).
The resinous block copolymers obtained according to method (1) do not show well balanced properties with respect to transparency, tensile strength, tensile elastic modulus and low-temperature impact strength when formed into a sheet. On the other hand, the resinous block copolymers according to method (2), when formed into the sheet, are superior in transparency, tensile strength and tensile elastic modulus to those of method (1). It has been, however, impossible to obtain sheets having a practically satisfactory low-temperature impact strength according to method (2).